1. Field of Invention
The present invention relates generally to extrusion blow molding machines having multiple accumulator heads and, more particularly, to a method for simultaneously controlling the rate at which the accumulator heads are filled with material and subsequently extrude a parison for the blow molding operation.
2. Description of the Related Art
In order to improve the production capacity of an extrusion blow molding machine, it is known in the art to use multiple accumulator heads to form multiple parisons for use in a particular blow molding application. The accumulators are typically supplied by a single extruder; although, in some cases, there may be a separate extruder for each accumulator. While the specific number of accumulators is determined by each application, the most common multiple accumulator machine configuration is the use of two accumulators supplied by one extruder in an extrusion blow molding machine.
In some situations, for example, the process involves extruding identical parisons from the two accumulators in order to form identical blow molded products. In other cases, there will be two different parts formed by the blow molding process, thus requiring extrusion of parisons that may be different in size, possibly diameter, wall thickness or overall length. In any case, in order to provide effective production operation, it is important that extrusion of the two parisons is completed simultaneously so that they are both ready for blow molding at the identical time. The timing is important so that the shape of one parison is not allowed to change (e.g., stretch) while waiting for the second accumulator to complete extrusion of the corresponding parison.
In an extrusion blow molding machine, the material supplied by the extruder occupies an internal space or chamber within the accumulator; the volume of the accumulator chamber is controlled by movement of a hydraulically operated ram or plunger. The ram moves downwardly to extrude a parison by forcing the material out of the chamber and through an annular die opening. The ram is forced upwardly during refill portion of the blow molding cycle as material is supplied by the extruder. Accordingly, it is also important that the accumulators be filled simultaneously so that they complete the filling process at the same time, even though they may ultimately contain different volumes of plastic material. The uniformity in filling helps assure consistency in material properties between the parisons, and enables the accumulators to begin extruding the parisons at the same point in time (without any delay), thereby reducing cycle time.
Without the capability for uniform timing of the multiple accumulator heads, it is virtually impossible to accomplish simultaneous movement of the accumulator rams over the same period even if their stroke is the same. The differences in the timing of the extrusion stroke or filling interval are attributable to a number of factors. For example, it is impractical to expect that the exact pressure conditions will apply to both accumulators, given the variations in the manufacture of the individual components that make up the accumulator assembly. In addition, there are other factors, such as the variance in temperatures, flow efficiencies and pressures of the hydraulic fluid that is used to move the ram in each accumulator head. Although the differences or variances by themselves may be relatively small, they combine to make it virtually impossible to assure precise coordination of timing of the ram movement between two accumulators, if they are independently controlled.
In order to accomplish more consistent timing with respect to extruding the parison and refilling multiple accumulator heads, prior art control systems for extrusion blow molding machines have attempted to compare the position and/or speed of the accumulator rams to coordinate the movement between the two accumulators (see FIG. 1). For example, U.S. Pat. No. 4,179,251 is specifically directed to this type of control over accumulator timing. In particular, the position of the ram of one accumulator would be compared to the position of the other ram during the extrusion of the parison or refilling of the accumulator chamber. If it was determined that one accumulator was filling (or extruding) faster than the other, the associated pressure valve would be adjusted to slow down the movement of the ram, thereby allowing the other accumulator to advance to a similar point in the process.
While this approach provides some improvement, its response capability is limited due to the delay inherent in accomplishing the steps involved in making the calculations and actuating the machine elements. Accordingly, the resulting adjustments to the movement of the ram are abrupt and can negatively effect the production quality of the blow molding process. In addition, the abrupt changes can result in abnormal wear on the hydraulic pressure control valves and the associated accumulator components. The preferred solution would provide for smooth, uniform filling in all accumulators, so that they reach their full shot position at exactly the same time, followed by simultaneous extrusion of the parisons so that the downward movement of all accumulator rams is completed at the same point in time.